A quantum key distribution system includes a transmitter, a receiver, and a quantum transmission path that connects the transmitter and the receiver. The transmitter transmits photons to the receiver via the quantum transmission path. Thereafter, the transmitter and the receiver mutually confirm signal information to share an encryption key. This technique is generally called quantum key distribution (QKD). According to the basic principles of quantum mechanics, the physical states of photons change when the photons are tapped by an eavesdropper on the quantum transmission path. This causes an error between the signal transmitted by the transmitter and the signal received by the receiver. QKD enables detection of the presence of an eavesdropper on the quantum transmission path by comparing common parts of the signals.
For QKD to work, it is desirable that the states of photons do not change throughout the quantum transmission path. However, photons change in both polarization and phase due to, e.g., a temperature change or vibration of the quantum transmission path. The quantum key distribution system includes a feedback mechanism that compensates for such a polarization disturbance and phase disturbance.
In addition, the QKD system may utilize wavelength multiplexing to improve the key generation rate (see, for example, Japanese Patent No. 5413687).
In a quantum communication system, such as a QKD system, utilizing phase modulation, modulators are important, but costly. In the optical communication system disclosed in Japanese Patent No. 5413687, a modulator is provided for each wavelength. Therefore, as the number of wavelengths increases, the number of modulators increases. As a result, the apparatus (transmitter and/or receiver) becomes complex and more costly.